It is known to deploy airbags to protect seat occupants in a collision event. It is desirable to optimize the deployment force of an airbag based on a determination that the seat is occupied by a large person, a small person, or a child seat. For example, it is proposed that the airbag deploys with normal force when the seat is occupied by a large person, deploys with reduced force when the seat is occupied by a small person, and does not deploy if a child seat is present in the seat. Some airbag controllers determine optimum deployment based upon a measured weight of the seat occupant. It is proposed that airbag controllers measuring the weight of the seat occupant perform a correction for seat belt tension in the determination process. For this purpose, it is proposed to provide a seat belt tension sensor within a seat belt assembly, typically within or connected to the buckle. However, including a separate tension sensor to determine seat belt tension undesirably adds cost and complexity to the seat belt assembly.
One type of seat belt assembly includes a retractor motor for retracting webbing and establishing a seat belt tension on the webbing. It is proposed to estimate seat belt tension by applying a voltage to the retractor motor, monitor the retractor motor for movement with a motor motion sensor, and estimate seat belt tension. Knowledge of motor motion is useful for estimating seat belt tension. However, including a separate motion sensor undesirably adds cost and complexity to the seat belt assembly.
What is needed is a reliable measure of a seat belt tension on a webbing in a seat belt assembly that includes a retractor motor which does not require a separate tension sensor or motor motion sensor, and thereby reduces the cost and complexity associated with determining seatbelt tension in a seat belt assembly.